I have an 8TB G-Raid w/ Thunderbolt which I use as a Time Machine backup.

Increasingly over the past couple of months, the icon would not appear on the desktop and occasionally I'd get the Disk Was Unmounted error...now the disk appears to be damaged as it will not mount on multiple computers, and it crashes Disk Utility when I attempt to Verify, Repair or Erase. Screen grab attached.

Upon rare occasions, the disk DOES mount (like as I'm writing this...!) and I am still not able to erase or reformat it (second screen grab.)

Thanks for posting. First off, you mentioned you have tried on multiple computers. Have you used a different thunderbolt cable with each computer or have you just tried with the same cable. If the same you may want to try with a different cable to make sure it is not providing a bad connection. I also noticed you used the erase tab with the raid slice drive. You could also use the partition tab and format free space to try removing the partitions if you can't delete the raid set itself. If that still isn't working then it is a problem with the internal hardware and can be replaced under warranty.

If it still does the same thing on a different thunderbolt cable RMA would be the best course of action. Please make sure you hang on to the thunderbolt cable as that would have been a separate purchase from the drive. You can file for a RMA here http://www.hgst.com/warranty/index_gtech_serial.do